Project Summary Neural circuits are composed of networks of specific cell types that are interconnected in precise patterns to give rise to normal brain function. Past studies have investigated the rates of connections that are formed by cultured neurons in vitro and these have revealed abnormalities both in a mouse model of autism (MeCP2 KO mice) and for human induced pluripotent stem cell (hiPSC)-derived neurons from schizophrenic patients (Brennand et al., 2011). But simple separation of cortical neurons into just excitatory versus inhibitory groups barely scratches the surface of the diversity of cortical cell types, the known specificity of connections between specific cell types in the abnormal or normal brain. In Project 5 we propose to develop and then implement novel tools that will allow assays of the cell-type specificity of connections that are formed in cultures of hiPSC- derived neurons. These assays are based on the use of glycoprotein deleted rabies viruses for trans-synaptic labeling of connected neurons, and conditional expression of EnvA/TVA targeting systems for directing the initial infection of rabies virus to specific cell types (Wickersham et al., 2007b; Marshel et al., 2010; Wall et al., 2010)